The present invention relates to a method for the preparation of a foamed body of a vinyl chloride-based resin composition or, more particularly, to an economical method for the continuous preparation of a foamed body of a vinyl chloride-based resin composition of high expansion having an outstandingly fine and uniform cellular structure in a continuous process.
In the prior art, several methods are known for the preparation of a foamed body of a vinyl chloride-based resin composition. For example, firstly, a vinyl chloride-based resin is admixed with a so-called decomposable blowing agent, which is a compound decomposable with heating to evolve a gaseous decomposition product, and the resin thus impregnated with the blowing agent is heated and shaped with simultaneous foaming by use of an extruder machine or injection machine. Secondly, a paste-like plastisol, which is a mixture of a vinyl chloride resin and a plasticizer, is foamed into a desired form by a mechanical means or foamed with simultaneous gelation of the resin composition by heating after admixing of a decomposable blowing agent. Thirdly, a resin composition containing a decomposable blowing agent is shaped into a desired form by a suitable means such as rolling at a temperature below the decomposition temperature of the blowing agent and then the thus shaped body is heated and foamed at a temperature sufficiently high for the decomposition of the blowing agent. Fourthly, a metal mold is filled with a mixture of a vinyl chloride resin and a decomposable blowing agent together with a volatilizable blowing agent, an organic solvent having a swelling power for the resin and a softening agent according to need, and the resin composition is melted and gelled in the metal mold by heating under pressure followed by solidification by cooling and the thus solidified resin composition is again heated to effect foaming under a reduced pressure.
Each of the above described methods, however, has its own disadvantages. For example, the first to the third methods above are not suitable for the preparation of a rigid or semi-rigid foamed body of high expansion. The fourth method is disadvantageous in the high costs for the preparation of the products because the method must be performed batch-wise and the process is rather complicated and lengthy taking a relatively long time.
The inventors have continued extensive investigations with an object to develop a method for the preparation of a foamed body of a vinyl chloride-based resin without the above mentioned problems and disadvantages in the prior art methods resulting in the development of an improved method disclosed in Japanese Patent Kokai No. 55-149328, according to which a resin composition obtained by formulating a vinyl chloride-based resin with a nucleating agent such as talc, an acrylic resin, a decomposable blowing agent such as an azodicarbonamide and the like together with a heat stabilizer is supplied to an extruder machine and the resin composition heated and gelled in the extruder machine is impregnated by pressurizing with a volatilizable blowing agent having a boiling point of 90.degree. C. or below which is an aliphatic hydrocarbon or a halogenated aliphatic hydrocarbon, such as propane, butane, pentane, methyl chloride, trichlorofluoromethane, dichlorotetrafluoroethane and the like and then extruded out of the extruder machine to effect foaming under reduced pressure.
This method, unfortunately, still has several problems that (1) the impregnation of the resin composition by pressurizing with the volatilizable blowing agent must be performed under an extremely high pressure and uniform dispersion or dissolution of the blowing agent in the resin composition is obtained only by giving a strong mechanical shearing to the resin composition and/or by decreasing the melt viscosity of the resin composition with heating at an increased temperature, (2) a relatively long dispersion and mixing zone is required for the extruder machine in order to obtain sufficiently uniform impregnation of the resin composition with the volatilizable blowing agent, (3) the temperature of the resin composition impregnated with the volatilizable blowing agent in the dispersion and mixing zone of the extruder must be somewhat decreased before the resin composition is extruded out of the machine since the resin composition in the dispersion and mixing zone of the extruder is at an excessively high temperature not suitable for extrusion as such, and (4) the extruder machine accordingly must be a large one with an elaborate design. Therefore, a practical barrier is given due to these problems for the industrialization of the above proposed method.